This instant invention is concerned with the elimination of polluting constituents from gaseous combustion products, e.g., flue gases. The invention is particularly directed toward the elimination of fugitive emission associated with the art of dry emissions filtration pollution control systems, a characteristic features of this invention is a method of continuous pollutant emissions filtration.
The problems of air pollution, resulting from the venting of combustion gases, is recognized as an interest of current research as dictated by the U.S. Environmental Protection Agency to improve the emission control of utilities and industrial operations. Among techniques used to separate fly ash and other pollutants from flue gases are a variety of gas filters, dynamic air-solids separators, wet scrubber, and electrostatic precipitators, although effective to a degree, substantial improvement is possible in the art of dry filtration techniques. There has been special regard to recovery efficiencies for particulate matter, heavy metals, and organic compounds associated with combustion emissions from coal fired power plants, incinerators of biomedical waste, and municipal wastes incinerators. Presently, dry process are approximately 65 to 85 percent effective in recovery of pollutant emission gases.
This invention improves the upon this dry process known as dry filtration emissions control that utilizes filter media in one or more divided chambers commonly termed a bag house chamber. This chamber is divided into three segmented areas e.g., bottom ash hopper, lower treating chamber, and upper treated chamber. A filter media exists between the upper and lower chambers separating a manifold waste gas inlet line fitted with a control valve to control the flow of the waste gas, and a manifold treated gas outlet.
Waste gas being processed is presently passed through a wet or dry injection chamber where different methods are used to improve absorption of gaseous pollutants in the gases passage from the injection point and the control chamber where pollutants are to a great extent captured by the filter media. This injection process has been shown to improve capture efficiency on an increased basis as ash and sorbent materials form on the surface of the filter media. As improved collection is key to the efficient to the operation of these systems, it is known that the efficiency of these injection systems is dependent upon the capture rate of the filter membrane on which these absorbing particles impinge and are ultimately captured in order to provide efficient recovery. It is known in present technology a method of cleaning the filer media that allows waste gas to pass through the filter media unaided by a collected sorbent layer. It is also known, that filter media operates most efficiently when a porous sorbent coating is accentual to provide protection from abrasion and chemical attack of the filter media, and also improves the filter media""ability to capture particles below 20 micron. It is also known in the art that sorbent applied to a filter media substrate is highly efficient in the removal of gaseous emissions to a very effective degree.
Current best available control technology or (BACT) is utilized in the cycle operations of filter chambers. These process provided in prior art uses on line and off line cleaning methods to eliminated capture pollutants and sorbent material from the treating chamber. In the case of xe2x80x9cOff Linexe2x80x9d cleaning the flow of waste gas to a treating chamber is terminated, and vibration or reverse air supply under pressure is used to remove these pollutants and sorbent coating from the surface of the filter media membrane separating the treated and untreated chambers. Once removal has been completed the chamber is cycled into an xe2x80x9cOn Linexe2x80x9d operation allowing the waste gas to flow through the filter media membrane at a reduced efficiency to capture pollutant gases due to the lack of a coating established on surface of the filter media. The other method in the art is commonly known as xe2x80x9cOn Linexe2x80x9d cleaning. This method is the preferred method of practice. In this method waste gas is not terminated during cleaning and continually flows while individual filter media segments are cleaned by the employment of backpressure valves that isolates the filter media from the exhaust manifold allowing compressed gases (air) to flow in a reverse flow fashion to release the coating from the surface of the filter media as described above once completed backpressure valves are reopened. It is also known that this method is problematic because when the filter segment is cleaned the waste gas flow through the uncoated filter media increases and permits more pollutants on a volume metric basis to pass untreated into the manifold treated exhaust chamber than from a coated filter. It is known that this increase in flow is caused by a corresponding decrease in flow resistance attributed to the coating residue accumulated upon the filter media substrate. In effect the coating is part of the filter mechanism. As such this instant invention is an apparatus that observes this coating as an essential component and maintains this coating through the waste processing cycle of the invention.
The Best Available Control Technology as defined by the U.S. Environmental Protection Agency (EPA) is therefore improved by this invention. The dry filtration process system is most frequently used to provide maximum control in the removal of harmful toxic emissions from entering the environment is therefore advanced.
The principal object of the instant invention is to promote progress in the aforementioned desiderata of the art and to provide a flue gas treatment process which is both economical of operation and potentially capable of producing returns on investment in the form of useful, reductions in waste, and sorbent usage. Especially, it is an object of this invention to provide a treatment readily adaptable to existing combustion facilities that provided full and uninterrupted control of pollutant emissions.
In accordance with the present invention, flue gases are typically passed through single or multiple filters constructed of a suitable porous material e.g., (cloth, metal, polymer composite, or ceramic), said filters are arranged in a manner having a common inlet and a common outlet bisected by a division member isolating the treated and untreated chambers from each other. This invention improves this prior art by surrounding the said filters on four sides in such a manner to provide an enclosure from adjacent filter segments. The enclosure is fitted with automatically operated dampers or isolation doors on the inlet and outlet sides. This adaptations allows for the; purge, cleaning, and precoating of the filter segments outside of the presents of a waste gas stream in the chamber.
For the purpose of this invention a row of filters is isolated above and below by double acting dampers. This invention also provides an improvement of prior art systems by the addition a heated clean sorbent air supply system to each chamber provided below the filter media and above the lower process waste stream isolation dampers. This improvement on prior art allows filter segments within the main chamber to be removed from service by closing the upper isolation dampers isolated the waste gas stream e.g., (off line condition) and through vibration or reverse flow remove filter coating in a no flow state, allowing fly ash and spent sorbent to drop into the fly ash hopper. Once coatings have been removed the chamber is purged until waste gas and dust have been removed then the lower dampers are closed and the upper dampers are opened. At this point a separated sorbent supply conveyed by clean heated air is allowed to enter the chamber through a control valve. As the heated conveying air with an entrained sorbent passes through the chamber the activated sorbent is deposited upon the surface of the filter media, this continues until a pressure drop is across the filter media is detected greater than a clean filter media measures. As in prior art a differential pressure switch is used to obtain these measurements. Once a pressure drop of sufficient amount is obtained, developed by the build up of a sorbent coatings the lower isolation dampers are opened, allowing process gas to pass through the filter media processed by a dense reactive chemical media on the surface of the filter. This process is repeated each time a filter or number of filter requires cleaning or when the systems is placed on line on startup. This invention permits the continuous processing of waste gas in a manner that provides maximum exposure of process gas to chemical sorbent and prevents fugitive waste gas emission from bypassing sorbent exposure as is demonstrated in prior art systems that use waste gas as the sorbent transport method.
The art of pollution control known as xe2x80x9cdryxe2x80x9d baghouse filtration is known to provide lower levels of efficiencies than xe2x80x9cwetxe2x80x9d process. This invention improves the dry filtration process to the same efficiencies as that established in wet process provided by a continuous mode of operation. It is known in the art that dry processes are not continuous in nature and are termed intermittent. It is also known that the dry process could operate at high levels of efficiency as that found in the wet process by resolving the intermittent behavior characteristic of the dry process. It is known that the intermittent processing during the processing cycle occurs after the cleaning of the filter media. Many prior art processes improve the intermittent behavior of these systems by injecting sorbent in the waste gas stream in the attempt to capture pollutants before by the sorbent prior to contact with the filter media. While this prior art practice improves the process, it is known that the most effective removal is obtained when the pollutants are in very close contact with a dense bed of sorbent material. It has been demonstrated that capture efficiency is a function of proximity. When chemicals or particulates are in close proximity with a sorbent supported on a substrate of filter media it is in this area that provides the most effective removal due to increased capture rates. Prior art process, while providing sorbent of different kinds and many forms of injection processes, do not address the intermittent cycle of a filter chamber or the root cause for reduced efficiency. It is known by the inventor that the root cause of the intermittent process is the use of waste gas as the source to transport sorbent to the filter media after the cleaning cycle has been complete. This invention improves this process by using a separate clean air stream to transport the sorbent to the filter media independent of the untreated process gas stream. This invention prevents polluted gases and particulates from escaping through uncoated filters and entering the atmosphere untreated by a sorbent.
Two useful benefits other than improved efficiency are obtained by the use of this invention""s continuous process.
It is known that sorbent chemicals are used in the operation of prior art systems, and that by increasing the volume of sorbent injected an improved pollutant removal is obtained. As increased restrictions in air quality are mandated by regulation, operators improve the efficiency of pollutant control systems by increasing the sorbent usage rates to obtain lower instantaneous pollution emission levels. As this invention provides a continuous process the use of sorbent is most often reduced and higher capture rates are experienced. As facility operators most often expect improved efficiency to cost more, the cost benefit of lower sorbent usage is welcomed. Another benefit of this process is a reduction of solid waste generated by excessive sorbent usage. The improved efficiency provided by this invention also provides additional productivity benefits. Lower sorbent usage translates into a reduction of transportation, and waste disposal cost.
In two tests preformed in 1992 at an operating incineration plant supervised by the Colorado Department of Air Quality this process improved the emissions control of a common baghouse filter from 62 percent to 99.9 percent and reduced sorbent usage by 35 percent.